1. Field of the Invention
The present invention relates to a polycarbonate resin composition. More particularly, the present invention is concerned with a polycarbonate resin composition comprising a specific polycarbonate resin comprising a plurality of aromatic polycarbonate main chains, wherein the a romantic polycarbonate main chains collectively contain specific heterounits in a specific amount in the polycarbonate main chains, and a rubber-reinforced thermoplastic resin The polycarbonate resin composition of the present invention has high impact resistance and high moldability.
2. Prior Art
Polycarbonates have been widely used in various fields as engineering plastics having excellent heat resistance, impact resistance and transparency. Production of polycarbonates has conventionally been conducted by using the phosgene process. However, polycarbonates produced by using the phosgene process have problems in that the production thereof needs the use of phosgene, which is poisonous, and that they contain residual methylene chloride (solvent), which not only adversely affects the thermal stability of the polycarbonates, but also causes corrosion of a mold used for molding the polycarbonates. Therefore, recently, polycarbonates produced by using the transesterification process, which are free from the problems accompanying polycarbonates produced by the phosgene process, have been drawing attention.
With respect to transesterification polycarbonates, it is known:
that almost colorless, transparent transesterification polycarbonates can be obtained on a laboratory scale; however, when the production of transesterification polycarbonates is conducted on a commercial scale, only those having slightly yellowish color can be obtained [see "Purasuchikku Zairyo Koza (5), Porikaboneto Jushi (Lecture on Plastic Materials (5), Polycarbonate Resins)", page 66, published in 1981 by The Nikkan Kogyo Shimbun Ltd., Japan], and PA1 that transesterification polycarbonates have disadvantages in that they have many branched structures, so that they have poor strength (danger of brittle fracture is high), as compared to phosgene process polycarbonates [see "Kobunshi (Polymer)", vol. 27, p. 521, July 1978)].
In order to alleviate these problems of the transesterification polycarbonates, various studies have been made on the structure and production process of the transesterification polycarbonates. With respect to the branched structures of the transesterification polycarbonates, it is known that such branched structures are formed as follows. During the progress of the polymerization reaction in the presence of an alkali in the reaction system, the polycarbonate chain being formed suffers a side reaction represented by the reaction formula described below, which is similar to the Kolbe-Schmitt reaction: ##STR1## As is apparent from the above-shown structure formed in the main chain by the side reaction, a branched chain grows and extends through ester bonds. In some cases, such a branched chain forms a crosslinked structure in the final polycarbonate [see "Purasuchikku Zairyo Koza (5), Porikaboneto Jushi (Lecture on Plastic Materials (5), Polycarbonate Resins)", page 64, published in 1981 by The Nikkan Kogyo Shimbun Ltd., Japan; and "Porikaboneto Jushi Hando Bukku (Polycarbonate Resin Hand Book)", page 49, published in 1992 by The Nikkan Kogyo Shimbun Ltd., Japan].
With respect to the structure of the transesterification polycarbonate, it has been attempted to reduce the amount of branched structure in the polycarbonate. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 5-105751 and Unexamined Japanese Patent Application Laid-Open Specification No. 5-202180 (corresponding to U.S. Pat. No. 5,468,836) disclose a technique to obtain a transesterification polycarbonate having no or almost no branched structure. Specifically, in these prior art documents, the transesterification reaction is conducted using a specific combination of catalysts, to thereby obtain a colorless polycarbonate having no or almost no branched structure which is formed by the side reaction during the polymerization. Unexamined Japanese Patent Application Laid-Open Specification No. 7-18069 (corresponding to U.S. Pat. No. 5,418,316) proposes a method for producing a polycarbonate, in which, by the use of a specific catalyst, the formation of the above-mentioned branched structure by the side reaction similar to the Kolbe-Schmitt reaction is suppressed to a level as low as 300 ppm or less. The polycarbonates disclosed in these prior art documents have high colorlessness; however, these polycarbonates have problems in that the non-Newtonian flow properties characteristic of a transesterificatlon polycarbonate decreases, so that the polycarbonates disadvantageously exhibit low-molding melt fluidity.
For solving the above problems, for example, Unexamined Japanese Patent Application Laid-Open Specification Nos. 5-271400 and 5-295101 (each corresponding to U.S. Pat. No. 5,468,836) disclose a transesterification technique in which the formation of the above-mentioned disadvantageous branched structure resulting from the side reaction of the above reaction formula is reduced by the use of a specific catalyst to thereby achieve an improvement in colorlessness of the formed polycarbonate, whereas the non-Newtonian flow properties of the polycarbonate are increased by intentionally introducing a specific other branched structure to the polycarbonate by the use of a multifunctional compound, so that the polycarbonate can be advantageously used for blow molding. Further, in U.S. Pat. No. 4,562,242, it is attempted to improve the molding melt fluidity of the polycarbonate by the use of a 5-(dimethyl-p-hydroxybenzyl)salicylic acid as a branching agent. However, the use of the multifunctional compound as mentioned above has problems in that the multifunctional compound promotes a crosslinking reaction during the polymerization, so that the final polycarbonate is likely to contain gel.
Therefore, it has been desired to develop a transesterification technique, in which the occurrence of branching of the polycarbonate structure can be controlled without using a multifunctional compound which is likely to cause gelation of the resultant polycarbonate, so as to produce a polycarbonate which not only has high colorlessness as well as high mechanical strengths but also exhibits high non-Newtonian flow properties, so that the polycarbonate can exhibit high molding melt fluidity, as compared to the phosgene process polycarbonates.
Further, with respect to the process for producing a transesterification polycarbonate, various improvements have been proposed. For example, with respect to a process in which use is made of a plurality of polymerizers which are connected in series, it has been proposed to use a special type of polymerizers as a final stage polymerizers, such as a special type of horizontal agitation type polymerizer (see Unexamined Japanese Patent Application Laid-Open Specification No. 2-153923) or a twin screw, vented extruder (see Examined Japanese Patent Application Publication No. 52-36159 and Unexamined Japanese Patent Application Laid-Open Specification No. 63-23926). However, the techniques of the above-mentioned prior art documents are only intended to promote the removal of phenol from the polymerization reaction system. Therefore, by these techniques, a polycarbonate having a high molecular weight can be easily obtained; however, the obtained polycarbonate is not satisfactory with respect to the properties thereof, such as mechanical properties and molding melt fluidity.
Polycarbonates have disadvantages in that the molding melt fluidity thereof is poor and the impact strength thereof is largely influenced by the thickness of a molded product obtained therefrom. By contrast, with respect to resin compositions comprising a polycarbonate and a rubber-reinforced thermoplastic resin, especially polycarbonate/ABS alloys, the molding melt fluidity is improved and the influence of the thickness of a molded product on the impact strength thereof is advantageously small. Due to this advantage, at present, resin compositions comprising a polycarbonate and a rubber-reinforced thermoplastic resin are used in a wide variety of applications. Further, in recent years, resin compositions comprising a polycarbonate and a rubber-reinforced thermoplastic resin are increasingly used in the field of the production of housings for hand-held personal computers, pocketable telephones and the like. However, resin compositions comprising a polycarbonate and a rubber-reinforced thermoplastic resin have a problem in that they have poor thermal stability and, hence, are likely to suffer marked discoloration when subjected to molding, so that the color of molded products disadvantageously varies. Resin compositions comprising a polycarbonate and a rubber-reinforced thermoplastic resin have another problem in that, when continuous molding of these resin compositions is conducted for a long period of time, corrosion occurs on the inner wall surface of a mold around a gas-releasing hole thereof, causing the molded products obtained to have less luster or to have different sizes from one another.
For solving the above-mentioned problems accompanying the resin compositions comprising a polycarbonate and a rubber-reinforced thermoplastic resin, many attempts have been made to improve the thermal stability of these resin compositions, especially polycarbonate/ABS alloys. For example, a method has been proposed in which various antioxidarts are incorporated in resin compositions comprising a polycarbonate and a rubber-reinforced thermoplastic resin when the resin compositions are subjected to extrusion or molding, thereby alleviating discoloration caused by heat degradation (see Unexamined Japanese Patent Application Laid-Open Specification No. 61-23640). However, these conventional attempts have been unable to satisfactorily improve the thermal stability of resin compositions comprising a polycarbonate and a rubber-reinforced thermoplastic resin, so that the above-mentioned problems due to the poor thermal stability of these resin compositions have not been solved. Further, these conventional techniques are unsatisfactory for preventing impact strength lowering and discoloration of the resin composition which are likely to occur when the resin composition experiences residence at high temperatures.
On the other hand, there have been attempts to improve the properties of a polycarbonate to be mixed with a rubber-reinforced thermoplastic resin. However, most of these attempts are simply intended to improve the mechanical properties of a resin composition comprising a polycarbonate and a rubber-reinforced thermoplastic resin by, for example, a method in which a multifunctional compound is used as a branching agent in the production of the polycarbonate. Almost no attempts have been made to solve the above-mentioned thermal stability problems accompanying resin compositions comprising a polycarbonate and a rubber-reinforced thermoplastic resin. Unexamined Japanese Patent Application Laid-Open Specification No. 4-239545 (corresponding to EP 496258), Japanese Patent Application prior-to-examination Publication (kohyo) No. 4-504137 (corresponding to WO90/10675), U.S. Pat. No. 4,677,162 and DE 3149812 disclose resin compositions comprising a branched polycarbonate and a rubber-reinforced resin, such as ABS. However, the branched polycarbonates disclosed in these prior art documents are those obtained using a multifunctional compound as a branching agent. Therefore, as described above in connection with the prior art concerning polycarbonates, a polycarbonate obtained using a multifunctional compound has a problem in that it is likely to contain gel. Further, resin compositions comprising a polycarbonate and a rubber-reinforced thermoplastic resin are still unsatisfactory in moldability.
For improving the thermal stability of an alloy of a transesterification polycarbonate and a rubber-reinforced resin, for example, Unexamined Japanese Patent Application Laid-Open Specification No. 5-239331 discloses a method in which an ABS resin is incorporated into a just-produced transesterification polycarbonate still in the molten state. This method is intended to prevent a thermal degradation of a polycarbonate by reducing the number of times the polycarbonate experiences thermal history and the number of times the polycarbonate suffers heat generated by shearing when a solid polycarbonate is melted and kneaded. However, this method cannot satisfactorily improve the thermal stability of such a polycarbonate resin composition.
Resin compositions comprising a polycarbonate and a rubber-reinforced thermoplastic resin are widely used for producing housings for, e.g., electrical appliances and various office automation machines, such as computers and word processors. For ensuring safety, materials for producing these housings are frequently required to have high flame retardancy. Further, in accordance with the recent remarkable advances in office automation machines, downsizing and miniaturization of these machines are also advancing in order to render them easily portable. Along with the downsizing and miniaturization, the thickness of housings is frequently required to be decreased for decreasing their weight. The decrease in the thickness of housings, in turn, requires that a polycarbonate resin composition used for molding have more improved properties with respect to both flame retardancy and moldability. However, the polycarbonate resin compositions of prior art cannot meet this need for high flame retardancy and high moldability.
On the other hand, in an application field (such as housings for a CRT and a copying machine) in which gathering of dust should be avoided, antistatic properties are important. For imparting antistatic properties to a molded product of a polycarbonate resin composition, in general, use is made of a method in which a water-absorptive compound, such as a polyalkylene oxide, or the like is incorporated into a resin composition to be molded, or a method in which a surfactant or the like is coated on a molded product. However, incorporation of a water-absorptive compound lowers the mechanical properties, heat resistance and the like of a resin composition. Coating of a surfactant or the like on a molded product is disadvantageous in that the imparted antistatic properties cannot be satisfactorily maintained.
A primary task of the present invention is to provide a polycarbonate resin composition, which has both high impact resistance and high moldability, containing a specific transesterification polycarbonate which is advantageous in that not only does it have high transparency and colorlessness as well as high mechanical strength, but also it exhibits high non-Newtonian flow properties, so that it can exhibit high molding melt fluidity. It is another task of the present invention to provide a polycarbonate resin composition which has not only high impact resistance and high moldability, but also the following additional advantageous properties the ability to prevent the dripping of a flaming molten resin particle upon being burnt; freedom from a lowering of impact resistance upon experiencing residence at high temperatures during processing; high discoloration resistance; and high antistatic properties.